Recovery of cooper from copper sulfide containing concentrates

ABSTRACT

Low grade copper sulfide containing concentrates and ores are treated with an alkaline cyanide, such as calcium cyanide and excess lime or sodium cyanide and excess sodium alkali. This dissolves the copper sulfide as a complex cyanide, the liquor is separated from the denuded solids, and the solids washed with water to remove adhering soluble copper cyanide complex. The two liquors are then treated with acid or metal salts to desulfurize them, transforming the sulfur content either to hydrogen sulfide or a metal sulfide and precipitating the copper as acid insoluble cuprous cyanide. The cuprous cyanide is separated and copper recovered therefrom by known methods, such as reduction with hot hydrogen. HCN evolved if the desulfurization is by acid and/or from the hydrogen-HCN produced by the hot hydrogen reduction is transformed into alkaline cyanide and reused. A reducing agent, such as sulfur dioxide, is present during the cuprous cyanide precipitation in sufficient amount to maintain copper in the cuprous state.

United States Patent [191 Cadwell l l Feb. 12, 1974 1 RECOVERY OF COOPERFROM COPPER Primary Examiner-Herbert T. Carter SULFIDE CONTAININGCONCENTRATES [75] Inventor: Edward Peter Cadwell, Tucson,

Ariz.

[73] Assignee: Treadwell Corporation, New York,

[22] Filed: Feb. 10, 1972 [21] Appl. No.: 225,095

[52] US. Cl 75/106, 75/105, 423/29, 423/47 [51] Int. Cl C22b 3/00 [58]Field of Search... 75/105, 106, 117; 423/29, 47

[56] References Cited UNITED STATES PATENTS 1,654,918 1/1928 Buchanan eta1. 75/106 X 3,303,021 2/1967 Roberts..... 75/105 3,321,303 5/1967Roberts..... 75/106 3,403,020 9/1968 Lower 75/106 3,189,435 6/1965Lowerm. 75/105 X 3,429,694 2/1969 Lower.... 75/105 X 1,810,487 6/1931Lawrnm' 423/29 Attorney, Agent, or FirmRobert Ames Norton [5 7] ABSTRACTLow grade copper sulfide containing concentrates and ores are treatedwith an alkaline cyanide, such as calcium cyanide and excess lime orsodium cyanide and excess sodium alkali. This dissolves the coppersulfide as a complex cyanide, the liquor is separated from the denudedsolids, and the solids washed with water to remove adhering solublecopper cyanide complex. The two liquors are then treated with acid ormetal salts to desulfurize them, transforming the sulfur content eitherto hydrogen sulfide or a metal sulfide and precipitating the copper asacid insoluble cuprous cya nide. The cuprous cyanide is separated andcopper recovered therefrom by known methods, such as reduction with hothydrogen. HCN evolved if the desulfurization is by acid and/or from thehydrogen-HCN produced by the hot hydrogen reduction is transformed intoalkaline cyanide and reused. A reducing agent, such as sulfur dioxide,is present during the cuprous cyanide precipitation in sufficient amountto maintain copper in the cuprous state.-

8 Claims, 1 Drawing Figure PAIENTEB Ha I 2 I974 Cu-CONC ENTRATES STEP 1CYANIDE LEACH B P'R O DJC T FILTER FLOTATION STEP 3 2 CLAUS DESULFURIZERPLANT NEUTRALIZE CYANIDE ABSORBER STEP 5 PRECIPITATION H CuCN STEP 6 rSTEP 8 FILTER HCN STRIPPER STEP 7 H CuCN REDUCTION CRYSTAI-I-IZER CuSTEP 9 CALCINER $0 OR CO2 RECOVERY OF COOPER FROM COPPER SULFIDECONTAINING CONCENTRATES BACKGROUND OF THE INVENTION Low grade coppersulfide containing ores and concentrates have been concentrated byflotation and smelted, with or without formation of copper matte. Theprocess, while economical with suitable materials, produces largequantities of sulfur dioxide, which is a serious atmospheric pollutantand if it is to be recovercd requires some fairly expensive treatment.The SO pollution problem is made much more serious if the concentratecontains iron sulfides, such as pyrite or the iron sulfide compoennt ofchalcopyrite. With many concentrates the amount of iron sulfides, whichwill be referred to in the remainder of this specification genericallyas pyrite though they do not necessarily always exist 100 percent in theform of this particular mineral, is a frequent and large contaminant ofthe ore or concentrate. In many cases the weight of pyrite may' be asmuch as 90 percent or more of-the total sulfide content of the ore orconcentrate. It is possible to separate, at least partially, sulfides ofcopper and iron, for example by selective froth flotation, but this addssubstantially to the cost and in the case of many ores, particularly lowgrade ores or concentrates with large amounts of pyrite, the cost forcopper recovery becomes prohibitive or at least unattractively high.Also, when there is a large amount of pyrite present a substantialamount of the copper sulfide may associate itself therewith and copperlosses are increased, which makes the recovery even less economicallyattractive, and in the case of some low grade copper sulfide ores,prohibitive.

In general pollutant legislation and economics have thus made therecovery of copper from certain low grade copper ores economicallyunfeasible and with some ores relatively expensive.

It is also possible with some very low grade copper ores to leach withsulfuric acid and produce solutions containing cupric sulfate. From suchsolutions there is now available a very economic process for recoveringpure copper. This forms the subject matter of the Roberts US. Pat. No.3,321,303, May 23, 1967. This process will be described more fully belowin connection with the improvements of the present invention. At thispoint, however, it should be pointed out that the production ofsolutions of cupric sulfate is still subject to substantial drawbacks,both economic and metallurgical. In the first place, acid leaching withordinary dilute sulfuric acid is only practical with oxidized copperores, and the recovery of copper leaves much to be desired.Nevertheless, where materials which would otherwise be worthless arconcerned, such as tailings piles, it can be a practical method.

It has been proposed to treat copper sulfide ores with an alkalinecyanide leach liquor. This term will be used throughout the presentspecification in its normal meaning in the art, namely a solution ofcyanide with sufficient alkali to keep the pH high enough, usually pH ormore, so that the copper is effecitvely dissolved and alkaline sulfidesare formed, Examples of alkaline cyanide liquors are'the black cyanideobtained from calcium cyanamid and which contains calcium cyanide and aconsiderable amount of lime, so that the requisite high pH is furnished.This black cyanide is very extensively used for the recovery of preciousmetals by the cyanidation process, and as it is sold at a lower price onthe basis of its sodium cyanide equivalent with no charge for the lime,it makes an attractive reagent. Sodium cyanide has also been used, withsuitable alkali to maintain the desired high pH, for certain operationswhere sodium cyanide is cheaply available or where extremely hightransportation costs render the black cyanide with its large limecontent unattractive. As will be pointed out below, the presentinvention uses such an alkaline cyanide solution in one ofits processsteps, and as it makes no difference whetherthis is based on calciumcyanide or sodium cyanide, the general term alkaline cyanide solution"will be used. The fact that alkaline cyanide solutions will dissolvecopper from copper-containing ores, such as those containing-coppersulfides, has been known for a long time. In fact the compounds areoften referred to as cyanicides, and their presence in precious metalores is a serious drawback and has rendered some such ores uneconomicfor the recovery of precious metals, either because of the largeconsumption of alkaline cyanide by the copper compounds or because ofthe cost of reducing the amount of cyanicides by other processes whichadd to the cost.

The use of alkaline cyanide solutions for the purpose of recoveringcopperfrom ores and concentrates containing copper sulfides and wherethe copper is the principal metal value is known, and such processes aredescribed in the Lower U.S. Pat. No. 3,189,435, June 15, 1965, and theRoberts U.S. Pat. No. 3,303,021, Feb. 7, 1967. Both of these patentsencountered the problem that a small but still not insignificant amountof copper is precipitated on acidification as cuprous cyanide, and thiswasconsidered as constituting a loss of cyanide, which had rendered thealkaline cyanide solution method of recovering of copper values hithertouneconomic with most ores and concentrates. The two patents referred toavoid serious losses from precipi tated cuprous cyanide by making surethat there is sufficient sulfide ions present when the solution of thecopper cyanide complex is broken up or copper precipitated therefrom.Typical compounds used are hydrogen sulfide, sodium sulfide, sodiumhydrosulfide, calcium sulfide, and in some casesammonium sulfide. Thecopper was largely precipitated as a sulfide, with cuprous cyanideprecipitation greatly reduced or substantially eliminated, and socyanide losses also reduced.

The Lower and Roberts processes above described might be consideredreally as processes which in the main are concentrating copper sulfidefrom low grade ores. The problems of recovering copper still remain: TheS0 pollution problemjis not solved, and while the high concentration ofcopper sulfide makes possible some practical recoveries in certaincases, the problem of getting copper. from copper sulfide even thoughgreatly concentrated and purified still remains.

SUMMARY OF THE INVENTION The present invention treats copper sulfidecontaining ores and concentrates with alkaline cyanide solutions just aswas done before. This solution is separated from the solids, which havebeen partially denuded of copper, just as was done before. However, withlow grade ores and hence an enormous amount of gangue including pyritein many ores, which are among those most effecitvely treated by thepresent invention, the copper adhering in solution to these largeamounts of gangue is a sufficiently large proportion of the total coppercontent of the ore or concentrate that it cannot be disregarded andstill have an economical process for low grade ores.

The next step after separating the copper cyanide complex solution fromthe denuded solids is to wash these solids with water. This has beendone before, but the wash water is so dilute that it could not be usedas a source of copper in the Lower and the earilier Roberts patentsreferred to above. They had to treat it in various ways to get asatisfactory concentration. In the present invention on the contrary,the wash water from the denuded solids is simply mixed with the alkalinecyanide leach liquors because, as will be pointed out below, the presentprocess can recover copper satisfactorily from this overall heavilydiluted solution. As a result a very large proportion of the originalcopper content of the ore or concentrate is recoverable, which makes thealkaline cyanidation treatment step economically feasible as theeventual cyanide loss is very drastically reduced to a pointsufficiently small so that the economics of the process are favorable.

At this point a very important feature of the present invention comesinto play. Instead of making every effort to prevent or reduceprecipitation of copper as cuprous cyanide and to precipitate as much aspossible as a copper sulfide, procedures are followed, which will bedescribed in more detail below, which desulfurize the liquor andprecipitate practically all or a very large portion of the dissolvedcopper as cuprous cyanide. The cuprous cyanide, except in the unusualsituations where there is a market for this chemical in high purity,

' can readily be treated by the process of the Roberts U.S. Pat. No.3,321,203 to obtain very pure copper with a very high recovery of HCN.This last step, of course, is not a new step as such, but when joined tothe other steps of the present invention as in one of the aspects of theinvention, it recovers copper with excellent recovery at lower cost,without pollution of the atmosphere with S and without significantlosses of cyanide. As in many inventions, the present invention involvesa combination of steps some of which taken by themselves are not new butare combined in a new way to produce a new and advantageous result. Thenovelty and patentability of the invention lies in the new combinationand the new result.

In the present invention themixed alkaline cyanide leach liquor and themuch more dilute wash liquor from the denuded solids are treated in aprocess which precipitates cuprous cyanide and not copper sulfide. Thisstep will be referred to below, particularly in conjunction with thedrawings, as a desulfuriz ing step. It may be accomplished in severalways: One way is to treat with acid. The sulfide of the particularalkali, such as calcium or sodium, which is formed when the coppersulfide is leached with the alkaline cyanide solution, is transformedinto a form from which H 8 can be stripped at a pH at which CuCN is notprecipitated, for example pH of approximately 8. The pH is then low.ered to removeHCN. During this operation cuprous cyanide precipitatespractically quantitatively. The removal of H 8 before precipitation ofCuCN is an essential feature of the present invention. If this is notdone the H 8 can react andprecipitate at least a part of the copper as asulfide.

Sulfur content in the above modification of the present invention istransformed into H 8, and the alkali of large amounts of pyritecontaminants. Not only is there the alkaline cyanide solution is, ofcourse, transformed into the salt of the acid used. The H S, of course,cannot be vented to the atmosphere as it is an even more toxic pollutantthan S0 However, it can readily be reacted with S0 which is eitherproduced in the process of the present invention or readily available,in a conventional Claus plant to form elemental sulfur, which is not apollutant and which can be shipped long dis- .tances economically.Another desulfurizing process is to react the alkaline sulfides producedin the alkaline cyanidation with a metal slat, suchas lead salt, whichforms the salt of the alkali cation of the sulfide and a sulfide of themetal, for example, lead sulfide. The importance of the step is that thesoluble sulfide is transformed into a form in which it is no longerreactively present in the liquor.

Treatment of the copper cyanide complex with acid is preferably effectedfirst by an acid, such as carbonic acid, down to a pH above that atwhich cuprous cyanide precipitates. The pH is sufficiently low, forexample approximating 8, so that H 3 is given off and is removed. Thenthe pH is lowered to that at which HCN is formed. The HCN can be removedand recovered in an alkaline hydroxide, which is then used to treat moreconcentrate or ore. The fixing of the HCN, of course, should bepreferably with a sufficient excess of alkali so that the resultingsolution has the right pH for treatment of the copper sulfide ores andconcentrates.

The pH is then maintained at a value at which precipitation of cuprouscyanide occurs, for example the pH may be as low as 5. This portion ofthe process should preferably be in the substantial absence of oxygen sothat the reactive copper compounds are in the cuprous state because itis only cuprous cyanide which is insoluble in the acid medium. Thecuprous cyanide produced is separated, for example by filtration,washed, dried, and if desired, briquetted. It can then be used in thelast step of the process of the Roberts U.S. Pat. No. 3,321,303 byreacting with hot hydrogen to produce very pure copper and a gas whichis a mixture of the excess hydrogen, always used in this portion of theRoberts process, and HCN. These constitutents are separated by knownmeans described in the Roberts patent and the HCN reused asmakeup HCNfor further amounts of alkaline cyanide leaching medium. The liquorremaining from the filtration in which the cuprous cyanide is recoveredcan then be steam stripped of any HCN content, evaporated, andcrystallized to recover sulfates, sulfites or carbonates of the metals,such as alkaline earth or akali metals, the particular salt of coursedepending on the acid used or salt used to desulfurize. The crystallizedmaterials can, if desired, be calcined to produce S0 CO an alkali suchas lime, and recycled in the. process. However, this is not an essentialstep, and the S0 or CO used may be obtained from any desired source. Itwill be noted that at no point is there any SO 2 discharged into theatmosphere, and the major content of sulfur in the ore or concentrate isdiscarded in the gangue, for example as pyrite in the preferred ores andconcentrates which contain no pollution, but little or'no expensivecyanide is lost. It should be noted that the precipitation of copper asthe sulfide in the Lower or first Roberts patent process doesnoteliminate completely cyanide losses, because as the two patents pointout it is not possible to prevent some of the copper precipitating ascuprous cyanide.

Every effort is made by the addition of the sulfide ion to reduce as faras possible any cuprous cyanide precipitate. As the patents point out,this is not a complete elimination, and this is one of the reasons thatthe Lower and first Roberts patent processes have not been economicallyattractive for many low grade copper sulfide containing ores andconcentrates. In the present process, by eliminating the precipitationof copper sulfides and, on the contrary, precipitating practically allof it as cuprous cyanide, it is possible to recover the vast majority ofthe cyanide and a much cheaper and pollution-free process results. Theprocess of the present invention, which does not minimize precipitationof cuprous cyanide as do the Lower and first Roberts process but, on thecontrary, maximizes it to an almost quantitative point, producesimportant advantages. It is not described or suggested in the Lower andfirst Roberts patents that by removing or avoiding the copper sulfideprecipitation, which is the essence of their processes, a moreeconomical process results and particularly, surprisingly, cyanidelosses are reduced to an insignificantly small amount. As has beenpointed out, after separating the alkaline cyanide leach liquor byfiltration and washing of the denuded solids, the presence of thesulfide ion is defeated. This step will be referred to asdesulfurization regardless of whether the sulfide ion is physicallyremoved as hydrogen sulfide or is transformed into a form in' which itno longer reacts with cuprous cyanide.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing is a flow sheet ofnumbered steps of a complete process which ends up as metallic copperand which recovers S0 and CO depending on the variant used. It willbenoted that essentially the only reagent which is consumed without moreor less complete recy- 1 cle is hydrogen. It should be noted that in theprocess 'of the second Roberts patent hydrogen is used in excess andthis excess is recycled.

While the flow sheet of the drawing with its nine steps sets forth thecomplete process using the variant of acid desulfurization, actuallysteps 6, 7, 8 and 9 are not new per se, or at least 7, 8 and 9, becausethe Roberts process in these steps is not changed. Once cuprous cyanideis precipitated in step 5 and separated in step 6, it may be used forany purpose desired, and one might consider that, at least as asub-combination, the present invention stops with step 5. However, if itis desired to recover metallic copper from the ore, step 7 makes thispossible without any cyanide loss. If the cuprous cyanide is used forsome other purpose, of course the large saving in HCN is not achievedand much more makeup HCN is needed. However, where pure cuprous cyanidehas a worthwhile market, as is the case in a few places in the world, itsells for more than the cost of the cyanide and the other steps used inproducing it.

From the standpoint of producing metallic copper, while step 7 by itselfis an old step in the second Roberts patent, it is one ofthe things thatmakes the present invention a greatly improved method of recoveringcopper because without this step the HCN consumed when copper isprecipitated as cuprous cyanide is not eliminated, the object which theLower and first Roberts patents so desperately tried to accomplish.Therefore, the overall process represented on the drawing, at leastthrough step 7, is also a part of the invention and included therein. Insuch a case the new desulfurizing step 3, which permits effectivelyusing the very dilute wash water from the denuded solids, is combinedwith two old steps, that is the preliminary leaching with alkalinecyanide solution and the final transformation ofthe cuprous cyanide intometallic copper.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will be describedin greater detail in conjunction with the following examples, which areillustrative only and in which the parts are by weight.

EXAMPLE 1 A froth flotation concentrate of a low grade copper orecontaining 0.85 percent copper, the concentrate also containing largeamounts of pyrite, accounting for substantially 97 percent of the totalsulfur content is used as a raw material. A 300 gram sample of therougher concentrate assaying 13.46% Cu and containing 40.38 gramsthereof was leached with 925 cc. of an alkaline leach liquor. Afterone-half hour agitation the slurry was filtered and the solids washed toremove adhering leach liquor therefrom. The two liquids were mixed. Inaddition to the copper present as cuprous sulfide, there was a smallamount of minerals which formed cyanide insoluble products. Theseincluded some molybdenum disulfide and a small amount of chalcopyrite.

The leach liquor contained 38.77 grams of dissolved copper as thecyanide complex and 1.63 grams was present in the solids as cyanideinsoluble minerals. These minerals were recovered from the denudedsolids by froth flotation, first with a molybdenite promoter to form amolybdenite concentrate and then with a promoter which floatschalcopyrite. This recovery from the solids is not an essential part ofthe present invention but is described merely to bring out that even thesmall amount of copper not dissolved, less than 4 percent, need not bewasted. Even if the solids were discarded, the copper recovery is betterthan 96 percent.

The leach liquor, at a pH of about 10, was treated with carbon dioxideto lower the pH to 8, thus expelling the H 8 from the solution. Additionof CO was continued to reduce the pH to about 7, during which time HCNwas evolved and fixed with alkali to form a part of the recylcedalkaline cyanide leach solution.

The leach liquor was then acidified with SO down to a pH of about 4.During this time cuprous cyanide was precipitated. The amount of SO wasapproximately equivalent to twice the stoichiometric quantity. Thecuprous cyanide precipitate was filtered from the slurry and dried. Therecovery of copper from the leach solution was practically quantitative.The precipitate assayed 70.33% Cu, which indicates pure cuprous cyanide.Only a faint trace of copper could be found in the filtrate. Thefiltrate, however, contained some HCN, which was stripped by steam andrecycled as shown on the flow sheet in the drawing.

It will be noted that the overall recovery of copper from the rougherconcentrate was very high, as pointed out above, even neglecting achalcopyrite concentrate from the solids. No 80,, was discharged to theatmosphere. The major portion of the sulfur was discharged in thedenuded product as pyrite and the remaining sulfur content was recoveredin the Claus plant as indicated on the drawing.

The-following table shows the metallurgical results with respect tocopper:

8 stripped of l-TCN as described in Example 1. The following table showsthe metallurgical results with re- TABLE 1 Assay content DistributionProduct Wt. Gr. Cu Gr. C. Cu.

Feed 300.0 13.46 40.38 100.0 Solution 2400 cc 16.14 gpl 38.75 96.01Residue 243.4 0.67 1.63 3.99 N0. 1 Con 6.4 1.11 0.07 0.17 No.2 Con 8.514.34 1.22 2.98 Tail 228.5 0.15 0.34 0.84 Filtrate 3000 cc. 0.07 gpl0.21 0.5 CuCN 54.74 70.4 38.54 95.44

EXAMPLE 2 Assays Content, Or. Distribution Product Wt. Gr. Wt. Cu Zn MoCu Mo Cu Mo Feed 1037.0 100.0 12.62 130.79 2.24- 100.0 100.0 Solution2.2 1. 54.14 119.02 91.00 Residue 739.9 76.2 1.49 11.77 9.00 Mo Con 3.10.3 0.64 0.09 36.69 0.02 1.13 0.02 504 Mo Tail 35.4 3.4 1.76 0.20 0.7080.62 0.27 0.47 12.1 Cu Con 39.3 3.8 13.22 0.54 0.882 5.19 0.34 3.96 15.2Cu Tail 712 1 68.7 0.82 0.11 0.07 5.94 0.50 4.54 22.3.

' EXAMPLE 2 spect to copper and molybdenum. The assay also shows Arougher concentrate was produced from an ore assaying 60% Cu as cuproussulfide. There was a major content of pyrite, amounting to approximately90 percent of the total sulfur content of the ore, together with somemolybdenite.

1037.0 grams of the roughter copper sulfide concentrate assaying 12.62%Cu and containing 130.79 gr. of copper was used. This was leached withan alkaline cyanide solution solubilizing 119.02 gr. of copper. As inExample 1 and as shown in the drawing, the slurry was filtered, thedenuded solids were washed the two solutions mixed, and the washed,denuded solids were then subjected to froth flotation to produce amolybdenum concentrate which contained 0.49 percent of the copper and acopper concentrate of cyanide insoluble copper minerals containing 3.96percent of the total copper. The tailing, pyrite, was rejected andcontained 4.54 percent of the total copper.

The leach liquor and wash was neutralized with CO to a pH of 8 and the H8 produced transformed into elemental sulfur by the conventional Clausprocess.

As in Example 1 the treatment with CO was continued down to a pH ofabout 7 and the HCN evolved trapped in alkali for recycling. Treatmentwith S0 as in the Example 1, was then started and continued to pH 4,using substantially the same amount of S0 As in Example 1 the cuprouscyanide was precipitated, filtered, dried, and assayed pure CuCn withonly a faint trace of copper in the filtrate. The filtrate was that theore contained a very small amount of zinc.

It will be noted that approximately 90 percent of the total sulfurcontent of the concentrate was rejected as pyrite and nearly all of theremaining 10 percent was transformed into elemental sulfur by the Clausprocess. No pollution of the atmosphere took place.

EXAMPLE 3 An ore assaying 0.85 percent copper as sulfide was subjectedto froth flotation, reground and cleaned, recovering approximately 80percent of the copper in the ore.

Nine hundred gr. of the final copper concentrate assaying 25.72% Cu andcontaining 231.48 gr. copper was leached with an alkaline'cyanidesolution of calcium cyanide and lime for 30 minutes and then filteredSufficient of the cyanide solution was used to solubilize about 90percent of the copper in the sulfide, 206.27 grams of copper beingsolubilized. The residue contained 10.9 percent of the copper .ascyanide insoluble concentrates, but when subjected to froth flotation asdescribed in conjunction with Examples 1 and 2,-the denuded solidscontained about 90 percent of the total sulfur content of the ore aspyrite. The concentrate also contained substantial amounts of molybdenumdisulfide, which, as in the other examples, was recovered by frothflotation of the denuded solids. The assays with respect to copper andmolybdenum are shown in the following table:

EXAMPLE 3 Assays Content, Gr. Distribution Product Wt. Gr. Wt. Cu Mo CuMo Cu Mo Feed 900.0 25.72 0.113 231.48 1.017 100.0 100.0 Solution 206.2789.10 17.4 Residue 623.9 69.3 4.04 25.21 0.85 10.90 83.6 Mo Con 13.6 1.517.14 4.9 2.33 0.66 1.0 64.9 Cu Con 81.8 9.1 26.61 0.11 21.77 0.09 9.49.0 Tail 528.5 58.7 0.21 0.019 1.11 0.10 0 4 9.8

EXAMPLE 4 Cuprous cyanide from the preceding three examples was treatedwith a stream of hydrogen passed over it at a temperature of 300C. Thetemperature was maintained for one hour and an excess of over 500percent of the stoichiometrical amount of hydrogen required was used. Astream containing 95 percent of the HCN was present and the copper ofextremely high purity represented essentially a quantitative yield fromthe cuprous cyanide used. The HCN and the excess hydrogen were thenseparated by conventional means and the HCN recycled to analkalinesolution and the hydrogen recycled with makeup hydrogen toreduce further amounts of cuprous cyanide to-metallic copper.

EXAMPLE 5 The pH was then adjusted to precipitate cuprous cyanide asdescribed in Example 1.

EXAMPLE 6 The procedure of Example 5 was repeated replacing the leadoxide with 4.9 grams of ZnO, which also represented a small excess overthe stoichiometrical. Zinc sulfide was produced, precipitated, andremoved by filtration. As in Example 5 no trace of sulfide sulfurremained in the solution.

I claim:

1. ln a process of recovering pure copper from copper sulfide-containingconcentrates and ores which comprises in combination the followingsteps,

a. leaching the material with an alkaline cyanide leaching medium inamounts sufficient to solubilize all of the reactive copper sulfide,

b. separating the leach liquid from the solids,

c. washing copper-containing solution from the solids and mixing it withthe leach liquid,

d. desulfurizing the leach and wash liquids by either treating with acidto a pH at which H 8 is set free but above that at which substantialprecipitation of copper sulfide takes place, or treating with a metalsalt other than a copper salt capable of reacting with the sulfidecontent of the leach liquid,

e. removing the sulfide sulfur compounds formed,

and

f. lowering the pH to the point where cuprous cyanide precipitation issubstantially complete, and separating the cuprous cyanide from theliquid.

2. A process according to claim 1 in which the cuprous cyanide isfiltered off and is subjected to reduction to metallic copper by meansof an excess of hydrogen at temperatures between C. and 600C, wherebythe cuprous cyanide is transformed into metallic copper and a mixture ofexcess hydrogen and HCN produced, and separating the hydrogen from theHCN and forming'alkaline cyanide leach medium from the HCN and recyclingit to leach more ore and recycling the hydrogen separated from the HCNafter heating to reduce further cuprous cyanide.

3. A process according to claim 1 in which the desulfurization iseffected by acid in two steps, maintaining the pH in the first stepsufficiently low to set free hydrogen sulfide, removing the hydrogensulfide, followed by lowering the pH to the point at which cuprouscyanide precipitates and HCN is evolved.

4. A process according to claim 3 in which the H 8 is set free by C0 andthe addition continued to about pH 8 and the H 8 is removed by strippingfrom the liquid 5. A process according to claim 4 in which the cuprouscyanide precipitation is effected by the addition of sulfurous acid.

6. A process according to claim 3 in which the hydrogen sulfide isreacted with SO by the Claus process to produce elemental sulfur.

7. A process according to claim 4 in which the hydrogen sulfide isreacted with SO by the Claus process to produce elemental sulfur.

8. A process according to claim 5 in which the hydrogen sulfide isreacted with SO by the Claus process to produce elemental sulfur.

2. A process according to claim 1 in which the cuprous cyanide isfiltered off and is subjected to reduction to metallic copper by meansof an excess of hydrogen at temperatures between 190*C. and 600*C.,whereby the cuprous cyanide is transformed into metallic copper and amixture of excess hydrogen and HCN produced, and separating the hydrogenfrom the HCN and forming alkaline cyanide leach medium from the HCN andrecycling it to leach more ore and recycling the hydrogen separated fromthe HCN after heating to reduce further cuprous cyanide.
 3. A processaccording to claim 1 in which the desulfurization is effected by acid intwo steps, maintaining the pH in the first step sufficiently low to setfree hydrogen sulfide, removing the hydrogen sulfide, followed bylowering the pH to the point at which cuprous cyanIde precipitates andHCN is evolved.
 4. A process according to claim 3 in which the H2S isset free by CO2 and the addition continued to about pH 8 and the H2S isremoved by stripping from the liquid.
 5. A process according to claim 4in which the cuprous cyanide precipitation is effected by the additionof sulfurous acid.
 6. A process according to claim 3 in which thehydrogen sulfide is reacted with SO2 by the Claus process to produceelemental sulfur.
 7. A process according to claim 4 in which thehydrogen sulfide is reacted with SO2 by the Claus process to produceelemental sulfur.
 8. A process according to claim 5 in which thehydrogen sulfide is reacted with SO2 by the Claus process to produceelemental sulfur.